Thermal spraying techniques have been used to apply durable coatings to metallic substrates. A wide variety of metallic alloys and ceramic compositions have been used in accordance with these prior art techniques.
The prior art thermal spray processes involve the generation of a high temperature carrier medium into which powders of the coating material are injected. With specific regard to plasma coating techniques, a plasma powder gun or a plasma wire gun is used in a controlled atmosphere to apply the coating to the substrate. The U.S. Pat. No. 4,235,943 to McComas et al, issued Nov. 25, 1980, and U.S. Pat. No. 4,256,779 to Sokol et al, issued Mar. 17, 1981, both patents being assigned the assignee of the present invention, relate to plasma spray methods and apparatus of the above described type. Such plasma spray coating guns as disclosed in these prior art patents utilize an inert gas, helium, as the plasma and carrier gas.
It is desirable in plasma spray methods to spray powders which are highly reactive. Specifically, the MCrAly family of coatings such as nickel, cobalt, chromium-aluminum ytrium alloys or cobalt nickel chromium-aluminum ytrium alloys can be used in which case oxygen caught within the coatings is a very critical factor to control. It is necessary to keep oxygen content in the spraying environment as low as possible. It is therefore a goal of the present invention to markedly reduce the oxide content of parts sprayed within the spraying environment by reducing oxides in the coating which tend to be detrimental.
Previously, low pressure plasma chambers have been used for plasma spraying in which the chamber is pumped down to a near vacuum and the parts are coated using a plasma gun process inside the chamber. These systems are very expensive, requiring large chambers for containing large parts which must be able to withstand the internal vacuum. Further, these chambers require mechanical pumps to create the vacuum within the chamber. These pumps contribute to the great expense of the use of the already expensive chambers.
The present invention provides a controlled atmosphere chamber which can markedly reduce the oxide content of parts sprayed thereby allowing the use of more reactive, very oxidation prone powders but not requiring the expense of using of a vacuum in the expense associated therewith.